Polymer additives used in color electrophoretic display medium

ABSTRACT

An electrophoretic medium may be incorporated into an electrophoretic display that includes a dispersion containing a non-polar fluid, a plurality of first charged colored particles, polyisobutylene, and an additive selected from co-polymers of vinyl aromatics and at least one hydrocarbon having 2 to 5 carbons and at least one double bond. The co-polymer and the ratio of the co-polymer to polyisobutylene is selected, such that bistability performance is maintained while improving the color state performance of the display.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application having Ser. No.15/893,747 filed on Feb. 12, 2018, which claims priority to and thebenefit of U.S. Provisional Application having Ser. No. 62/459,174,filed on Feb. 15, 2017, the contents of which are incorporated byreference herein in their entireties.

BACKGROUND OF INVENTION

This invention relates to color electrophoretic displays and polymeradditives that promote bistability of electrophoretic media in the colordisplays without degrading color performance.

Particle-based electrophoretic displays have been the subject of intenseresearch and development for a number of years. In such displays, aplurality of charged particles (sometimes referred to as pigmentparticles) move through a fluid under the influence of an electricfield. The electric field is typically provided by a conductive film ora transistor, such as a field-effect transistor. Electrophoreticdisplays have good brightness and contrast, wide viewing angles, statebistability, and low power consumption when compared with liquid crystaldisplays. Such electrophoretic displays have slower switching speedsthan LCD displays, however, and electrophoretic displays are typicallytoo slow to display real-time video. Additionally, the electrophoreticdisplays can be sluggish at low temperatures because the viscosity ofthe fluid limits the movement of the electrophoretic particles. Despitethese shortcomings, electrophoretic displays can be found in everydayproducts such as electronic books (e-readers), mobile phones and mobilephone covers, smart cards, signs, watches, shelf labels, and flashdrives.

An electrophoretic image display (EPID) typically comprises a pair ofspaced-apart plate-like electrodes. At least one of the electrodeplates, typically on the viewing side, is transparent. Anelectrophoretic fluid composed of a dielectric solvent with chargedpigment particles dispersed therein is enclosed between the twoelectrode plates. An electrophoretic fluid may have one type of chargedpigment particles dispersed in a solvent or solvent mixture of acontrasting color. In this case, when a voltage difference is imposedbetween the two electrode plates, the pigment particles migrate byattraction to the plate of polarity opposite that of the pigmentparticles. Thus, the color showing at the transparent plate can beeither the color of the solvent or the color of the pigment particles.Reversal of plate polarity will cause the particles to migrate to theopposite plate, thereby reversing the color. Alternatively, anelectrophoretic fluid may have two types of pigment particles ofcontrasting colors and carrying opposite charges and the two types ofpigment particles are dispersed in a clear solvent or solvent mixture.In this case, when a voltage difference is imposed between the twoelectrode plates, the two types of pigment particles would move toopposite ends (top or bottom) in a display cell. Thus, one of the colorsof the two types of the pigment particles would be seen at the viewingside of the display cell.

Numerous patents and applications assigned to or in the names of theMassachusetts Institute of Technology (MIT), E Ink Corporation, E InkCalifornia, LLC and related companies describe various technologies usedin encapsulated and microcell electrophoretic and other electro-opticmedia. In a microcell electrophoretic display, the charged pigmentparticles are retained within a plurality of cavities formed within acarrier medium, typically a polymeric film. The technologies describedin these patents and applications include:

(a) Electrophoretic particles, fluids and fluid additives; see forexample U.S. Pat. Nos. 5,961,804; 6,017,584; 6,120,588; 6,120,839;6,262,706; 6,262,833; 6,300,932; 6,323,989; 6,377,387; 6,515,649;6,538,801; 6,580,545; 6,652,075; 6,693,620; 6,721,083; 6,727,881;6,822,782; 6,831,771; 6,870,661; 6,927,892; 6,956,690; 6,958,849;7,002,728; 7,038,655; 7,052,766; 7,110,162; 7,113,323; 7,141,688;7,142,351; 7,170,670; 7,180,649; 7,226,550; 7,230,750; 7,230,751;7,236,290; 7,247,379; 7,277,218; 7,286,279; 7,312,916; 7,375,875;7,382,514; 7,390,901; 7,411,720; 7,473,782; 7,532,388; 7,532,389;7,572,394; 7,576,904; 7,580,180; 7,679,814; 7,746,544; 7,767,112;7,848,006; 7,903,319; 7,951,938; 8,018,640; 8,115,729; 8,119,802;8,199,395; 8,257,614; 8,270,064; 8,305,341; 8,361,620; 8,363,306;8,390,918; 8,582,196; 8,593,718; 8,654,436; 8,902,491; 8,961,831;9,052,564; 9,114,663; 9,158,174; 9,341,915; 9,348,193; 9,361,836;9,366,935; 9,372,380; 9,382,427; and 9,423,666; and U.S. PatentApplications Publication Nos. 2003/0048522; 2003/0151029; 2003/0164480;2003/0169227; 2003/0197916; 2004/0030125; 2005/0012980; 2005/0136347;2006/0132896; 2006/0281924; 2007/0268567; 2009/0009852; 2009/0206499;2009/0225398; 2010/0148385; 2011/0217639; 2012/0049125; 2012/0112131;2013/0161565; 2013/0193385; 2013/0244149; 2014/0011913; 2014/0078024;2014/0078573; 2014/0078576; 2014/0078857; 2014/0104674; 2014/0231728;2014/0339481; 2014/0347718; 2015/0015932; 2015/0177589; 2015/0177590;2015/0185509; 2015/0218384; 2015/0241754; 2015/0248045; 2015/0301425;2015/0378236; 2016/0139483; and 2016/0170106;

(b) Microcell structures, wall materials, and methods of formingmicrocells; see for example U.S. Pat. Nos. 7,072,095 and 9,279,906;

(d) Methods for filling and sealing microcells; see for example U.S.Pat. Nos. 7,144,942 and 7,715,088;

(e) Films and sub-assemblies containing electro-optic materials; see forexample U.S. Pat. Nos. 6,982,178 and 7,839,564;

(f) Backplanes, adhesive layers and other auxiliary layers and methodsused in displays; see for example U.S. Pat. Nos. 7,116,318 and7,535,624;

(g) Color formation and color adjustment; see for example U.S. Pat. Nos.7,075,502 and 7,839,564;

(h) Methods for driving displays; see for example U.S. Pat. Nos.7,012,600 and 7,453,445;

(i) Applications of displays; see for example U.S. Pat. Nos. 7,312,784and 8,009,348; and

(j) Non-electrophoretic displays, as described in U.S. Pat. No.6,241,921 and U.S. Patent Applications Publication No. 2015/0277160; andapplications of encapsulation and microcell technology other thandisplays; see for example U.S. Patent Application Publications Nos.2015/0005720 and 2016/0012710.

Many commercial electrophoretic media essentially display only twocolors, with a gradient between the black and white extremes, known as“grayscale.” Such electrophoretic media either use a single type ofelectrophoretic particle having a first color in a colored fluid havinga second, different color (in which case, the first color is displayedwhen the particles lie adjacent the viewing surface of the display andthe second color is displayed when the particles are spaced from theviewing surface), or first and second types of electrophoretic particleshaving differing first and second colors in an uncolored fluid. In thelatter case, the first color is displayed when the first type ofparticles lie adjacent the viewing surface of the display and the secondcolor is displayed when the second type of particles lie adjacent theviewing surface). Typically the two colors are black and white.

If a full color display is desired, a color filter array may bedeposited over the viewing surface of the monochrome (black and white)display. Displays with color filter arrays rely on area sharing andcolor blending to create color stimuli. The available display area isshared between three or four primary colors such as red/green/blue (RGB)or red/green/blue/white (RGBW), and the filters can be arranged inone-dimensional (stripe) or two-dimensional (2×2) repeat patterns. Otherchoices of primary colors or more than three primaries are also known inthe art. The three (in the case of RGB displays) or four (in the case ofRGBW displays) sub-pixels are chosen small enough so that at theintended viewing distance they visually blend together to a single pixelwith a uniform color stimulus (‘color blending’).

Although seemingly simple, electrophoretic media and electrophoreticdevices display complex behaviors. For instance, it has been discoveredthat simple “on/off” voltage pulses are insufficient to achievehigh-quality text in electronic readers. Rather, complicated “waveforms”are needed to drive the particles between states and to assure that thenew displayed text does not retain a memory of the previous text, i.e.,a “ghost.” See, for example, U.S. Patent Application No. 20150213765.Compounded with the complexities of the electric fields, the internalphase, i.e., the mixture of particles (pigment) and fluid, can exhibitunexpected behavior due to interactions between charged species and thesurrounding environment (such as an encapsulation medium) upon theapplication of an electric field. Additionally, unexpected behaviors mayresult from impurities in the fluid, pigments, or encapsulation medium.Accordingly, it is difficult to predict how an electrophoretic displaywill respond to variations in the internal phase composition.

It has been found, for example in U.S. Pat. No. 7,170,670, that theaddition of certain polymers, such as polyisobutylene, to the suspendingfluid used in electrophoretic displays provides an increase in imagestability, i.e. bistability, greater than can be accounted for by theincrease in viscosity of the fluid caused by the addition of thepolymer. Accordingly, the use of these polymers in the suspending fluidallows for substantial increases in image stability without excessiveincrease in the switching time of the display. However, it has beenfound that the introduction of these polymers results in a degradationof the color state when used in colored electrophoretic media.

Thus, there is a need for improved electrophoretic media and displayshaving improved bistability without sacrificing color state performance.

SUMMARY OF INVENTION

It is an aspect of the present invention to provide an electrophoreticmedium that includes a non-polar fluid, a plurality of at least a firstset of charged colored particles, polyisobutylene, and an additiveselected from co-polymers of vinyl aromatics and at least onesubstituted or unsubstituted hydrocarbon having 2 to 5 carbons and atleast one double bond. The co-polymer and the ratio of the co-polymer topolyisobutylene is selected, such that bistability performance ismaintained while improving the color state performance of the display.

This illustrative embodiment is mentioned not to limit or define thedisclosure, but to provide an example to aid understanding thereof.Additional embodiments are discussed in the Detailed Description, andfurther description is provided there. Advantages offered by one or moreof the various embodiments may be further understood by examining thisspecification or by practicing one or more embodiments presented.

The electrophoretic medium of the invention may be encapsulated, forexample in a microcell or a protein coacervate, as discussed in theBackground section. In addition, electrophoretic media of the inventioncan be dispersed in a polymer matrix. The encapsulated orpolymer-dispersed electrophoretic media may be incorporated into a frontplane laminate (FPL) and/or electro-optic displays as discussed in theBackground. Such materials can be used to create electrophoretic imagedisplays (EPID), signs, or architectural materials that will changeappearance upon receipt of a signal.

BRIEF DESCRIPTION OF THE FIGURE

FIG. 1 is a graph plotting the red state (Red a*) values versus theconcentration of polyisobutylene using the data provided in Table 1.

DETAILED DESCRIPTION

The performance of various types of electrophoretic media can beimproved by including the combination of additives described herein. Forexample, the combination of polyisobutylene with a copolymer accordingto the invention can improve the bistability of the electrophoreticmedia, as well as maintain or improve the color state performance whencompared to the color performance of electrophoretic media lacking thecopolymer.

The terms bistable and bistability are used herein in their conventionalmeaning in the art to refer to displays comprising display elementshaving first and second display states differing in at least one opticalproperty, and such that after any given element has been driven, bymeans of an addressing pulse of finite duration, to assume either itsfirst or second display state, after the addressing pulse hasterminated, that state will persist for at least several times, forexample at least four times, the minimum duration of the addressingpulse required to change the state of the display element. It is shownin U.S. Pat. No. 7,170,670 that some particle-based electrophoreticdisplays capable of gray scale are stable not only in their extremeblack and white states but also in their intermediate gray states, andthe same is true of some other types of electro-optic displays. Thistype of display is properly called multi-stable rather than bistable,although for convenience the term bistable may be used herein to coverboth bistable and multi-stable displays.

In one embodiment of the present invention, an electrophoretic mediumcomprises a dispersion. The dispersion may comprise, a non-polar fluid,a plurality of first charged particles, polyisobutylene, and an additiveselected from co-polymers of vinyl aromatics and at least onesubstituted or unsubstituted hydrocarbon having 2 to 5 carbons and atleast one double bond.

To ensure that the polyisobutylene and the copolymer are notsubstantially absorbed on to the electrophoretic particles, theadditives and suspending non-polar fluid combination should desirably besuch that the fluid is at least a theta solvent for polyisobutylene andthe copolymer at room temperature. In order to extend the temperaturerange of the enhanced image stability, it is optimal if the fluid is atleast a moderately good to good solvent for the image stabilityadditives. One method of choosing an appropriate additive/fluidcombination is to plot the intrinsic viscosity of the combinationagainst the molecular weight of the additives; desirable combinationsare those in which the slope of a best linear fit of the log of theintrinsic viscosity against the log of the molecular weight is at least0.5, and desirably in the range of about 0.55 to about 0.8.

Typically, the suspending fluid used in electrophoretic displays is anon-polar aliphatic hydrocarbon (alone or in combination with ahalogenated hydrocarbon) and with such fluids, it is preferred that theadditives are hydrocarbon polymers essentially free from functionalgroups, such as ionizable or ionic substituents, that may cause theadditive to interact with chemical sites on the electrophoreticparticles, or to physically adsorb to the surfaces of theelectrophoretic particles.

For electrophoretic mediums with aliphatic hydrocarbon/halogenatedhydrocarbon suspending fluids, the additives are preferably acombination of a copolymer, as further described below, andpolyisobutylene (poly-2-methylpropene). A variety of other types ofpolymers for example polysiloxanes, and in particularpolydimethylsiloxane, may also optionally be used. Poly isobutylene iscommercially available in molecular weight ranges appropriate for use inthe present invention, inexpensive, stable at high temperatures,resistant to oxidation and does not contain easily reactive or ionizablecomponents. As already mentioned, ionic or ionizable components areundesirable in the polymer since release of such components into thesuspending fluid might cause undesirable changes in the charges on theelectrophoretic particles, and thus upon their electrophoreticmobilities. The polyisobutylene desirably has a number average molecularweight in excess of 100,000, and preferably in the range of about150,000 to about 3,000,000, and a weight average molecular weight inexcess of 100,000 and preferably in the range of about 300,000 to about3,000,000; similar molecular weight ranges may be used with otherpolymers. Polyisobutylenes falling within these molecular weight rangesare readily available commercially, for example from Sigma-Aldrich,Inc., P.O. Box 2060, Milwaukee Wis. 53201 under catalogue numbers18145-5 and 18146-3. The polyisobutylene also desirably has a viscosityaverage molecular weight in the range of about 200,000 to 1,200,000g/mole. As noted above, the copolymer that may be included in theadditive combination for the electrophoretic media made according to thevarious embodiments of the present invention may be a copolymer of avinyl aromatic and a substituted or unsubstituted hydrocarbon having 2to 5 carbons and at least one double bond. The vinyl aromatics that maybe included in the co-polymer include, but are not limited to styrene,alpha-methyl styrene, 2-methyl styrene, 3-methyl styrene, 4-methylstyrene, 44-butyl styrene, 4-cyclohexyl styrene, 4-dodecyl styrene,2,4-diisopropyl styrene, 2,4,6-trimethyl styrene, 2-ethyl-4-benzylstyrene, 4-(phenyl butyl)styrene, 1-vinyl naphthalene, 2-vinylnaphthalene, vinyl anthracene, 4-methoxy styrene, monochlorostyrene,dichlorostyrene, divinyl benzene, indene, methyl indene, andcombinations thereof. Styrene is preferred. Examples of a substituted orunsubstituted hydrocarbons having 2 to 5 carbons and at least one doublebond include, but are not limited to, ethylene, propylene, isoprene,butylene, and butadiene. In one embodiment, it is preferred that thecopolymer is the polymerization product of styrene, ethylene, andpropylene.

The copolymer is preferably non-absorbing on the particles and maycomprise a diblock copolymer having a first block soluble in the fluidand a second block not swellable by the fluid. The diblock copolymer maycomprise a first polystyrene block and a second block of a polyalkene,for example polyisoprene. Such block copolymers are commerciallyavailable and include, for example aggregating diblock copolymers madefrom styrene and poly(ethylenepropylene), such as Kraton (RegisteredTrade Mark) G1701, G1702 or G1730, all available from Kraton Polymers,Inc, Belpre, Ohio).

The presence of a substantial proportion of aromatic groups may causepoor solubility or adsorption of an additive onto the electrophoreticparticles when the suspending fluid is an equal part by weight mixtureof an aliphatic hydrocarbon and halogenated hydrocarbon. Therefore, aratio of vinyl aromatic monomeric units to alkene monomeric units shouldbe selected for the copolymer additive, so that an unwanted effect onthe suspending fluid is avoided. The copolymers preferably contain fromabout 10 to about 50 percent, more preferably 20 to about 40 percent,styrene by weight.

The amount of additive for improving bistability and color performancein any specific system varies with the degree of image stabilityrequired, the molecular weight of the additives, and the maximumswitching time of the medium which can be tolerated. However, by way ofgeneral guidance, it is desirable that the combined additives (i.e. thecombined weight of polyisobutylene and copolymer) be added in an amountof from about 0.1 to about 2.5 percent by weight of the suspendingfluid, and preferably in an amount of about 0.5 to about 1 percent byweight. Below about 0.1 percent, depending upon the molecular weight ofthe polymer, the polymer produces little or no increase in imagestability other than that attributable to the increase in the viscosityof the suspending fluid, while polymer concentrations above about 2.5percent cause an increase in viscosity which may render switching timesunacceptable. The weight ratio of polyisobutylene to copolymer in thedispersion may be 1:5 to 5:1, more preferably 1:3 to 3:1.

The electrophoretic medium of the invention may include one or moretypes of charged particles. For purposes of this invention, a particleis any component that is charged or capable of acquiring a charge (i.e.,has or is capable of acquiring electrophoretic mobility), and, in somecases, this mobility may be zero or close to zero (i.e., the particleswill not move). For example, the electrophoretic medium may includefirst, second, third, fourth, fifth or sixth, types of chargedparticles. The particles may vary in charge, density, hydrophobicityand/or zeta potential. Various combinations of particle groups may haveopposite charge polarities relative to each other. Also, the particlesmay have different colors, such as magenta, red, orange, yellow, green,cyan, blue, violet, black, and white. For example in one embodiment ofthe present invention, an electrophoretic medium may comprise aplurality of first, second, and third charged particles dispersed in anon-polar fluid, wherein the first charged particles are magenta, red,yellow, green, cyan, or blue, the second charged particles are white andhave a charge opposite of the charge of the first charged particles, andthe third charged particles are black and have a charge similar to thecharge of the first charged particles.

The particles used in the various embodiments of the present inventionmay also optionally include colorless or transparent particles. Theelectrophoretic medium may additionally include surfactants, such asionic surfactants, i.e., surfactants having a quaternary amineheadgroup.

There is much flexibility in the choice of particles already known tothose skilled in the art of electrophoretic displays. Typicalconsiderations for the electrophoretic particle are its opticalproperties, electrical properties, and surface chemistry. The pigmentshould also be insoluble in the suspending fluid. The particles may besurface treated so as to improve charging or interaction with a chargingagent, or to improve dispersability. The particles may be, for example,inorganic particles, neat pigments, dyed (laked) pigments orpigment/polymer composites, or any other component that is charged orcapable of acquiring a charge.

Examples of inorganic pigments include, but are not limited to, TiO₂,ZrO₂, ZnO, Al₂O₃, CI pigment black 26 or 28 or the like (e.g., manganeseferrite black spinel or copper chromite black spinel). Particles, suchas titania particles may be coated with a metal oxide, such as aluminumoxide or silicon oxide, for example.

Useful neat pigments include, but are not limited to, PbCrO₄, Cyan blueGT 55-3295 (American Cyanamid Company, Wayne, N.J.), Cibacron Black BG(Ciba Company, Inc., Newport, Del.), Cibacron Turquoise Blue G (Ciba),Cibalon Black BGL (Ciba), Orasol Black BRG (Ciba), Orasol Black RBL(Ciba), Acetamine Black, CBS (E. I. du Pont de Nemours and Company,Inc., Wilmington, Del., hereinafter abbreviated “du Pont”), CroceinScarlet N Ex (du Pont) (27290), Fiber Black VF (du Pont) (3023S), LuxolFast Black L (du Pont) (Solv. Black 17), Nirosine Base No. 424 (du Pont)(50415 B), Oil Black BG (du Pont) (Solv. Black 16), Rotalin Black RM (duPont), Sevron Brilliant Red 3 B (du Pont); Basic Black DSC (DyeSpecialties, Inc.), Hectolene Black (Dye Specialties, Inc.), AzosolBrilliant Blue B (GAF, Dyestuff and Chemical Division, Wayne, N.J.)(Solv. Blue 9), Azosol Brilliant Green BA (GAF) (Solv. Green 2), AzosolFast Brilliant Red B (GAF), Azosol Fast Orange RA Conc. (GAF) (Solv.Orange 20), Azosol Fast Yellow GRA Conc. (GAF) (13900 A), Basic BlackKMPA (GAF), Benzofix Black CW-CF (GAF) (35435), Cellitazol BNFV ExSoluble CF (GAF) (Disp. Black 9), Celliton Fast Blue AF Ex Conc (GAF)(Disp. Blue 9), Cyper Black IA (GAF) (Basic Black 3), Diamine Black CAPEx Conc (GAF) (30235), Diamond Black EAN Hi Con. CF (GAF) (15710),Diamond Black PBBA Ex (GAF) (16505); Direct Deep Black EA Ex CF (GAF)(30235), Hansa Yellow G (GAF) (11680); Indanthrene Black BBK Powd. (GAF)(59850), Indocarbon CLGS Conc. CF (GAF) (53295), Katigen Deep Black NNDHi Conc. CF (GAF) (15711), Rapidogen Black 3 G (GAF) (Azoic Black 4);Sulphone Cyanine Black BA-CF (GAF) (26370), Zambezi Black VD Ex Conc.(GAF)(30015); Rubanox Red CP-1495 (The Sherwin-Williams Company,Cleveland, Ohio) (15630); Raven 11 (Columbian Carbon Company, Atlanta,Ga.), (carbon black aggregates with a particle size of about 25 μm),Statex B-12 (Columbian Carbon Co.) (a furnace black of 33 μm averageparticle size), and chrome green.

Laked pigments are particles that have a dye precipitated on them orwhich are stained. Lakes are metal salts of readily soluble anionicdyes. These are dyes of azo, triphenylmethane or anthraquinone structurecontaining one or more sulphonic or carboxylic acid groupings. They areusually precipitated by a calcium, barium or aluminum salt onto asubstrate. Typical examples are peacock blue lake (Cl Pigment Blue 24)and Persian orange (lake of Cl Acid Orange 7), Black M Toner (GAF) (amixture of carbon black and black dye precipitated on a lake).

A dark particle of the dyed type may be constructed from any lightabsorbing material, such as carbon black, or inorganic black materials.The dark material may also be selectively absorbing. For example, a darkgreen pigment may be used. Black particles may also be formed bystaining latices with metal oxides, such latex copolymers consisting ofany of butadiene, styrene, isoprene, methacrylic acid, methylmethacrylate, acrylonitrile, vinyl chloride, acrylic acid, sodiumstyrene sulfonate, vinyl acetate, chlorostyrene,dimethylaminopropylmethacrylamide, isocyanoethyl methacrylate andN-(isobutoxymethacrylamide), and optionally including conjugated dienecompounds such as diacrylate, triacrylate, dimethylacrylate andtrimethacrylate. Black particles may also be formed by a dispersionpolymerization technique.

The electrophoretic media according to the various embodiments of thepresent invention may be incorporated into displays, or into front planelaminates or inverted front plane laminates that are coupled to abackplane to make a display. Many of the aforementioned patents andapplications recognize that dispersions containing the pigments andadditives described above may be encapsulated in microcapsules, forexample. Furthermore, the walls surrounding the discrete microcapsulesin an encapsulated electrophoretic medium could be replaced by acontinuous phase, thus producing a so-called polymer-dispersedelectrophoretic display, in which the electrophoretic medium comprises aplurality of discrete droplets of an electrophoretic fluid and acontinuous phase of a polymeric material, and that the discrete dropletsof electrophoretic fluid within such a polymer-dispersed electrophoreticdisplay may be regarded as capsules or microcapsules even though nodiscrete capsule membrane is associated with each individual droplet;see for example, U.S. Pat. No. 6,866,760. Accordingly, for purposes ofthe present application, such polymer-dispersed electrophoretic mediaare regarded as sub-species of encapsulated electrophoretic media. Arelated type of electrophoretic display is a so-called microcellelectrophoretic display. In a microcell electrophoretic display, thecharged particles and the fluid are not encapsulated withinmicrocapsules but instead are retained within a plurality of cavitiesformed within a carrier medium, typically a polymeric film. See, forexample, U.S. Pat. Nos. 6,672,921 and 6,788,449, both assigned to SipixImaging, Inc.

An encapsulated electrophoretic display typically does not suffer fromthe clustering and settling failure mode of traditional electrophoreticdevices and provides further advantages, such as the ability to print orcoat the display on a wide variety of flexible and rigid substrates.(Use of the word printing is intended to include all forms of printingand coating, including, but without limitation: pre-metered coatingssuch as patch die coating, slot or extrusion coating, slide or cascadecoating, curtain coating; roll coating such as knife over roll coating,forward and reverse roll coating; gravure coating; dip coating; spraycoating; meniscus coating; spin coating; brush coating; air knifecoating; silk screen printing processes; electrostatic printingprocesses; thermal printing processes; ink jet printing processes;electrophoretic deposition (See U.S. Pat. No. 7,339,715); and othersimilar techniques.) Thus, the resulting display can be flexible.Further, because the display medium can be printed (using a variety ofmethods), the display itself can be made inexpensively.

The aforementioned U.S. Pat. Nos. 6,672,921, 6,788,449, and 6,866,760describe methods of assembling electrophoretic displays. Essentially,this patent describes a laminate comprising a light-transmissiveelectrically-conductive layer and a layer of a solid electro-opticmedium in electrical contact with the electrically-conductive layer.Typically, the light-transmissive electrically-conductive layer will becarried on a light-transmissive substrate, which is preferably flexible,in the sense that the substrate can be manually wrapped around a drum(say) 10 inches (254 mm) in diameter without permanent deformation. Theterm light-transmissive is used in this patent and herein to mean thatthe layer thus designated transmits sufficient light to enable anobserver, looking through that layer, to observe the change in displaystates of the electro-optic medium, which will normally be viewedthrough the electrically-conductive layer and adjacent substrate (ifpresent); in cases where the electro-optic medium displays a change inreflectivity at non-visible wavelengths, the term light-transmissiveshould of course be interpreted to refer to transmission of the relevantnon-visible wavelengths. The substrate will typically be a polymericfilm, and will normally have a thickness in the range of about 1 toabout 25 mil (25 to 634 μm), preferably about 2 to about 10 mil (51 to254 μm). The electrically-conductive layer is conveniently a thin metalor metal oxide layer of, for example, aluminum or indium tin oxide(ITO), or may be a conductive polymer. Poly(ethylene terephthalate)(PET) films coated with aluminum or ITO are available commercially, forexample as aluminized Mylar (Mylar is a Registered Trade Mark) from E.I.du Pont de Nemours & Company, Wilmington Del., and such commercialmaterials may be used with good results in the front plane laminate.

Assembly of an electro-optic display may be effected by attaching theabove-described laminate to a backplane with an adhesive underconditions effective to cause the adhesive layer to adhere to thebackplane, thereby securing the adhesive layer, layer of electro-opticmedium and electrically-conductive layer to the backplane. This processis well-adapted to mass production since the front plane laminate may bemass produced, typically using roll-to-roll coating techniques, and thencut into pieces of any size needed for use with specific backplanes.

In addition to the additives of the invention, electrophoretic media mayalso include charge control agents (CCAs). For example, pigmentparticles may be functionalized or surface coated with charged orchargeable groups. The CCAs may be absorbed into the particles, they maybe covalently bound to the surface of the particles, and they may existin a charge complex, or be loosely associated via van der Waals forces.Charge control agents often charge the particles by poorly understoodand uncontrolled processes, and can lead to undesirably highconductivity of the electrophoretic medium. Also, because the chargecontrol agent is only physically adsorbed on to the particles and is notbound thereto, changes in conditions may cause partial or completedesorption of the charge control agent from the particles, withconsequent undesirable changes in the electrophoretic characteristics ofthe particles. The desorbed charge control agent might resorb on toother surfaces within the electrophoretic medium, and such resorptionhas the potential for causing additional problems.

Charge control agents comprising a quaternary amine and an unsaturatedpolymeric tail comprising monomers of at least 10 carbon atoms in lengthare preferred. Quaternary amines comprise a quaternary ammonium cation[NR₁R₂R₃R₄]⁺ bonded to an organic molecule, for example an alkyl groupor an aryl group. Quaternary amine charge control agents typicallyinclude a long non-polar tail attached to the charged ammonium cation,such as the families of fatty acid quaternary amines offered by AkzoNobel under the tradenames ARQUAD. The quaternary amine charge controlagents may be purchased in a purified form, or the charge control agentsmay be purchased as a reaction product that has formed a quaternaryamine charge control agent. For example, SOLSPERSE 17000 (LubrizolCorporation), may be purchased as a reaction product of12-hydroxy-octadecanoic acid homopolymer withN,N-dimethyl-1,3-propanediamine and methylbisulfate. Other useful ioniccharge control agents include, but are not limited to, sodiumdodecylbenzenesulfonate, metal soap, polybutene succinimide, maleicanhydride copolymers, vinylpyridine copolymers, vinylpyrrolidonecopolymer, (meth)acrylic acid copolymers orN,N-dimethylaminoethyl(meth)acrylate copolymers), Alcolec LV30 (soylecithin), Petrostep B100 (petroleum sulfonate) or B70 (bariumsulfonate), OLOA 11000 (succinimide ashless dispersant), OLOA 1200(polyisobutylene succinimides), Unithox 750 (ethoxylates), Petronate L(sodium sulfonate), Disper BYK 101, 2095, 185, 116, 9077 & 220 andANTITERRA series.

The charge control agents may be added to the electrophoretic medium ata concentration of greater than 1 g of charge control agent for every100 g of charged particles. For example, the charge control agent tocharged particle ratio may be 1:30 (wt/wt), e.g., 1:25 (wt/wt), e.g.,1:20 (wt/wt). The charge control agents may have an average molecularweight of greater than 12,000 grams/mole, e.g., greater than 13,000grams/mole, e.g., greater than 14,000 grams/mole, e.g., greater than15,000 grams/mole, e.g., greater than 16,000 grams/mole, e.g., greaterthan 17,000 grams/mole, e.g., greater than 18,000 grams/mole, e.g.,greater than 19,000 grams/mole, e.g., greater than 20,000 grams/mole,e.g., greater than 21,000 grams/mole. For example, the average molecularweight of the charge control agent may be between 14,000 grams/mole and22,000 grams/mole, e.g., between 15,000 grams/mole and 20,000grams/mole. In some embodiments, the charge control agents have anaverage molecular weight of about 19,000 grams/mole.

Additional charge control agents may be used, with or without chargedgroups in polymer coatings, to provide good electrophoretic mobility tothe electrophoretic particles. Stabilizers may be used to preventagglomeration of the electrophoretic particles, as well as prevent theelectrophoretic particles from irreversibly depositing onto the capsulewall. Either component can be constructed from materials across a widerange of molecular weights (low molecular weight, oligomeric, orpolymeric), and may be a single pure compound or a mixture. An optionalcharge control agent or charge director may be used. These constituentstypically consist of low molecular weight surfactants, polymeric agents,or blends of one or more components and serve to stabilize or otherwisemodify the sign and/or magnitude of the charge on the electrophoreticparticles. Additional pigment properties which may be relevant are theparticle size distribution, the chemical composition, and thelightfastness.

In addition to solubility, as already indicated, the non-polarsuspending fluid containing the particles should be chosen based onproperties such as density and refractive index. A preferred suspendingfluid has a low dielectric constant (about 2), high volume resistivity(about 1015 ohm-cm), low viscosity (less than 5 centistokes (“cst”)),low toxicity and environmental impact, low water solubility (less than10 parts per million (“ppm”)), and a low refractive index (less than1.2).

The choice of non-polar fluid may be based on concerns of chemicalinertness, density matching to the electrophoretic particle, or chemicalcompatibility with both the electrophoretic particle and boundingcapsule (in the case of encapsulated electrophoretic displays). Theviscosity of the fluid should be low when movement of the particles isdesired.

Non-polar organic solvents, such as halogenated organic solvents,saturated linear or branched hydrocarbons (e.g. C6-C18 branched alkanesor C7-C10 branched alkanes), silicone oils, and low molecular weighthalogen-containing polymers are some useful non-polar fluids. Thenon-polar fluid may comprise a single fluid. The non-polar fluid will,however, often be a blend of more than one fluid in order to tune itschemical and physical properties. Furthermore, the non-polar fluid maycontain additional surface modifiers to modify the surface energy orcharge of the electrophoretic particle or bounding capsule. Reactants orsolvents for the microencapsulation process (oil soluble monomers, forexample) can also be contained in the suspending fluid. Additionalcharge control agents can also be added to the suspending fluid.

Useful organic solvents include, but are not limited to, epoxides, suchas decane epoxide and dodecane epoxide; vinyl ethers, such as cyclohexylvinyl ether and Decave (Registered Trade Mark of International Flavors &Fragrances, Inc., New York, N.Y.); and aromatic hydrocarbons, such astoluene and naphthalene. Useful halogenated organic solvents include,but are not limited to, tetrafluorodibromoethylene, tetrachloroethylene,trifluorochloroethylene, 1,2,4-trichlorobenzene and carbontetrachloride. These materials have high densities. Useful hydrocarbonsinclude, but are not limited to, dodecane, tetradecane, the aliphatichydrocarbons in the Isopar (Registered Trade Mark) series (Exxon,Houston, Tex.), Norpar (Registered Trade Mark) (a series of normalparaffinic liquids), Shell-Sol (Registered Trade Mark) (Shell, Houston,Tex.), and Sol-Trol (Registered Trade Mark) (Shell), naphtha, and otherpetroleum solvents. These materials usually have low densities. Usefulexamples of silicone oils include, but are not limited to, octamethylcyclosiloxane and higher molecular weight cyclic siloxanes, poly(methylphenyl siloxane), hexamethyldisiloxane, and polydimethylsiloxane. Thesematerials usually have low densities. Useful low molecular weighthalogen-containing polymers include, but are not limited to,poly(chlorotrifluoroethylene) polymer (Halogenated Hydrocarbon Inc.,River Edge, N.J.), Galden (Registered Trade Mark) (a perfluorinatedether from Ausimont, Morristown, N.J.), or Krytox (Registered TradeMark) from du Pont (Wilmington, Del.). In a preferred embodiment, thesuspending fluid is a poly (chlorotrifluoroethylene) polymer. In aparticularly preferred embodiment, this polymer has a degree ofpolymerization from about 2 to about 10. Many of the above materials areavailable in a range of viscosities, densities, and boiling points.

In some embodiments, the non-polar fluid will include an opticallyabsorbing dye. This dye must be soluble in the fluid, but will generallybe insoluble in the other components of the capsule. There is muchflexibility in the choice of dye material. The dye can be a purecompound, or blends of dyes to achieve a particular color, includingblack. The dyes can be fluorescent, which would produce a display inwhich the fluorescence properties depend on the position of theparticles. The dyes can be photoactive, changing to another color orbecoming colorless upon irradiation with either visible or ultravioletlight, providing another means for obtaining an optical response. Dyescould also be polymerizable by, for example, thermal, photochemical orchemical diffusion processes, forming a solid absorbing polymer insidethe bounding shell.

A number of dyes already known to those skilled in the art ofelectrophoretic displays will prove useful. Useful azo dyes include, butare not limited to: the Oil Red dyes, and the Sudan Red and Sudan Blackseries of dyes. Useful anthraquinone dyes include, but are not limitedto: the Oil Blue dyes, and the Macrolex Blue series of dyes. Usefultriphenylmethane dyes include, but are not limited to, Michler's hydrol,Malachite Green, Crystal Violet, and Auramine O.

Particle dispersion stabilizers may also be added to prevent particleflocculation or attachment to the capsule walls. For the typical highresistivity liquids used as suspending fluids in electrophoreticdisplays, non-aqueous surfactants may be used. These include, but arenot limited to, glycol ethers, acetylenic glycols, alkanolamides,sorbitol derivatives, alkyl amines, quaternary amines, imidazolines,dialkyl oxides, and sulfosuccinates.

Encapsulation of the internal phase may be accomplished in a number ofdifferent ways. Numerous suitable procedures for microencapsulation aredetailed in both Microencapsulation, Processes and Applications, (I. E.Vandegaer, ed.), Plenum Press, New York, N.Y. (1974) and Gutcho,Microcapsules and Microencapsulation Techniques, Noyes Data Corp., ParkRidge, N.J. (1976). The processes fall into several general categories,all of which can be applied to the present invention: interfacialpolymerization, in situ polymerization, physical processes, such ascoextrusion and other phase separation processes, in-liquid curing, andsimple/complex coacervation.

Numerous materials and processes should prove useful in formulatingdisplays of the present invention. Useful materials for simplecoacervation processes to form the capsule include, but are not limitedto, gelatin, poly(vinyl alcohol), poly(vinyl acetate), and cellulosicderivatives, such as, for example, carboxymethylcellulose. Usefulmaterials for complex coacervation processes include, but are notlimited to, gelatin, acacia, carageenan, carboxymethylcellulose,hydrolyzed styrene anhydride copolymers, agar, alginate, casein,albumin, methyl vinyl ether co-maleic anhydride, and cellulosephthalate. Useful materials for phase separation processes include, butare not limited to, polystyrene, poly(methyl methacrylate) (PMMA),poly(ethyl methacrylate), poly(butyl methacrylate), ethyl cellulose,poly(vinylpyridine), and polyacrylonitrile. Useful materials for in situpolymerization processes include, but are not limited to,polyhydroxyamides, with aldehydes, melamine, or urea and formaldehyde;water-soluble oligomers of the condensate of melamine, or urea andformaldehyde; and vinyl monomers, such as, for example, styrene, methylmethacrylate (MMA) and acrylonitrile. Finally, useful materials forinterfacial polymerization processes include, but are not limited to,diacyl chlorides, such as, for example, sebacoyl, adipoyl, and di- orpoly-amines or alcohols, and isocyanates. Useful emulsion polymerizationmaterials may include, but are not limited to, styrene, vinyl acetate,acrylic acid, butyl acrylate, t-butyl acrylate, methyl methacrylate, andbutyl methacrylate.

Capsules produced may be dispersed into a curable carrier, resulting inan ink which may be printed or coated on large and arbitrarily shaped orcurved surfaces using conventional printing and coating techniques.

In the context of the present invention, one skilled in the art willselect an encapsulation procedure and wall material based on the desiredcapsule properties. These properties include the distribution of capsuleradii; electrical, mechanical, diffusion, and optical properties of thecapsule wall; and chemical compatibility with the internal phase of thecapsule.

The capsule wall generally has a high electrical resistivity. Althoughit is possible to use walls with relatively low resistivity, this maylimit performance in requiring relatively higher addressing voltages.The capsule wall should also be mechanically strong (although if thefinished capsule powder is to be dispersed in a curable polymeric binderfor coating, mechanical strength is not as critical). The capsule wallshould generally not be porous. If, however, it is desired to use anencapsulation procedure that produces porous capsules, these can beovercoated in a post-processing step (i.e., a second encapsulation).Moreover, if the capsules are to be dispersed in a curable binder, thebinder will serve to close the pores. The capsule walls should beoptically clear. The wall material may, however, be chosen to match therefractive index of the internal phase of the capsule (i.e., thesuspending fluid) or a binder in which the capsules are to be dispersed.For some applications (e.g., interposition between two fixedelectrodes), monodispersed capsule radii are desirable.

An encapsulation technique that is suited to the present inventioninvolves a polymerization between urea and formaldehyde in an aqueousphase of an oil/water emulsion in the presence of a negatively charged,carboxyl-substituted, linear hydrocarbon polyelectrolyte material. Theresulting capsule wall is a urea/formaldehyde copolymer, whichdiscretely encloses the internal phase. The capsule is clear,mechanically strong, and has good resistivity properties.

The related technique of in situ polymerization utilizes an oil/wateremulsion, which is formed by dispersing the electrophoretic fluid (i.e.,the dielectric liquid containing a suspension of the pigment particles)in an aqueous environment. The monomers polymerize to form a polymerwith higher affinity for the internal phase than for the aqueous phase,thus condensing around the emulsified oily droplets. In one in situpolymerization process, urea and formaldehyde condense in the presenceof poly(acrylic acid) (see, e.g., U.S. Pat. No. 4,001,140). In otherprocesses, described in U.S. Pat. No. 4,273,672, any of a variety ofcross-linking agents borne in aqueous solution is deposited aroundmicroscopic oil droplets. Such cross-linking agents include aldehydes,especially formaldehyde, glyoxal, or glutaraldehyde; alum; zirconiumsalts; and polyisocyanates.

The coacervation approach also utilizes an oil/water emulsion. One ormore colloids are coacervated (i.e., agglomerated) out of the aqueousphase and deposited as shells around the oily droplets through controlof temperature, pH and/or relative concentrations, thereby creating themicrocapsule. Materials suitable for coacervation include gelatins andgum arabic. See, e.g., U.S. Pat. No. 2,800,457.

The interfacial polymerization approach relies on the presence of anoil-soluble monomer in the electrophoretic composition, which once againis present as an emulsion in an aqueous phase. The monomers in theminute hydrophobic droplets react with a monomer introduced into theaqueous phase, polymerizing at the interface between the droplets andthe surrounding aqueous medium and forming shells around the droplets.Although the resulting walls are relatively thin and may be permeable,this process does not require the elevated temperatures characteristicof some other processes, and therefore affords greater flexibility interms of choosing the dielectric liquid.

Additional materials may be added to encapsulated medium to improve theconstruction of an electrophoretic display. For example, coating aidscan be used to improve the uniformity and quality of the coated orprinted electrophoretic ink material. Wetting agents may be added toadjust the interfacial tension at the coating/substrate interface and toadjust the liquid/air surface tension. Wetting agents include, but arenot limited to, anionic and cationic surfactants, and nonionic species,such as silicone or fluoropolymer-based materials. Dispersing agents maybe used to modify the interfacial tension between the capsules andbinder, providing control over flocculation and particle settling.

In other embodiments, the electrophoretic medium may be contained inmicrofabricated cells, i.e., microcells, such as fabricated by E Inkunder the tradename MICROCUP. Once the microcells are filled with theelectrophoretic medium, the microcells are sealed, an electrode (or anelectrode array) is affixed to the microcells, and the filled microcellsare driven with electric fields to create a display.

For example, as described in U.S. Pat. No. 6,930,818, a male mold may beused to imprint a conductive substrate, upon which is formed atransparent conductor film. A layer of a thermoplastic or thermosetprecursor is then coated on the conductor film. The thermoplastic orthermoset precursor layer is embossed at a temperature higher than theglass transition temperature of the thermoplastic or thermoset precursorlayer by the male mold in the form of a roller, plate or belt. Onceformed, the mold is released during or after the precursor layer ishardened to reveal an array of microcells. The hardening of theprecursor layer may be accomplished by cooling, cross-linking byradiation, heat or moisture. If the curing of the thermoset precursor isaccomplished by UV radiation, UV may radiate onto the transparentconductor film from the bottom or the top of the web as shown in FIGS.2a and 2b of U.S. Pat. No. 6,930,818. Alternatively, UV lamps may beplaced inside the mold. In this case, the mold must be transparent toallow the UV light to radiate through the pre-patterned male mold on tothe thermoset precursor layer.

The thermoplastic or thermoset precursor for the preparation of themicrocells may be multifunctional acrylate or methacrylate, vinylether,epoxide and their oligomers, polymers and the like. A crosslinkableoligomer imparting flexibility, such as urethane acrylate or polyesteracrylate, is usually also added to improve the flexure resistance of theembossed micro-cups. The composition may contain polymer, oligomer,monomer and additives or only oligomer, monomer and additives.

In general, the microcells can be of any shape, and their sizes andshapes may vary. The microcells may be of substantially uniform size andshape in one system. However, in order to maximize the optical effect,microcells having a mixture of different shapes and sizes may beproduced. For example, microcells filled with a dispersion of the redcolor may have a different shape or size from the green microcells orthe blue microcells. Furthermore, a pixel may consist of differentnumbers of microcells of different colors. For example, a pixel mayconsist of a number of small green microcells, a number of large redmicrocells, and a number of small blue microcells. It is not necessaryto have the same shape and number for the three colors.

The openings of the microcells may be round, square, rectangular,hexagonal, or any other shape. The partition area between the openingsis preferably kept small in order to achieve a high color saturation andcontrast while maintaining desirable mechanical properties. Consequentlythe honeycomb-shaped opening is preferred over, for example, thecircular opening.

For reflective electrophoretic displays, the dimension of eachindividual microcell may be in the range of about 10² to about 5×10⁵μm², preferably from about 10³ about 5×10⁴ μm². The depth of themicrocells is in the range of about 3 to about 100 microns, preferablyfrom about 10 to about 50 microns. The opening to wall ratio is in therange of from about 0.05 to about 100, preferably from about 0.4 toabout 20. The distances of the openings usually are in the range of fromabout 15 to about 450 microns, preferably from about 25 to about 300microns from edge to edge of the openings.

Taken together, it will be apparent to those skilled in the art thatnumerous changes and modifications can be made in the specificembodiments of the invention described above without departing from thescope of the invention. Accordingly, the whole of the foregoingdescription is to be interpreted in an illustrative and not in alimitative sense.

EXAMPLES

Examples are now given, though by way of illustration only, to showdetails of preferred electrophoretic media of the present invention.

To demonstrate the effect of polyisobutylene (PIB) on the colorperformance of a display, four dispersions, CS1, CS2, CS3, and CS4(Comparative Examples), were prepared containing PIB at differentconcentrations.

The four samples of electrophoretic ink media contained 30% polymercoated titanium oxide particles, 7 wt % of red particles and 8% blackparticles, PIB, 0.2% Solsperse 19000 and other charge adjuvants inisoparaffin solvent. Table 1 provides the concentrations of PIB includedin each sample.

The electrophoretic media was sealed between two transparent ITO-PETelectrodes through a microcell filling-sealing technique to provide fourdisplay samples. The test samples were driven by a waveform generatorusing the same driving sequence. Measurement of the L*a*b* opticalperformance were conducted using an X-rite iOne spectrophotometer undera D65 illuminance setting.

TABLE 1 CS1 CS2 CS3 CS4 Polyisobutylene (wt. %) 0.0% 0.30% 0.60% 0.90%Red L* 33.9 31.9 31.0 33.5 Red a* 52.2 48.1 43.4 38.3

The Red a* (Ra) values exhibited by the display and provided in Table 1were plotted in FIG. 1 demonstrating a decrease with increasingconcentrations of PIB.

Three samples of electrophoretic media were prepared using the samemethod for preparing the Comparative Examples, except that astyrene-ethylene-propylene copolymer was added to the dispersions. Theoptical testing method was repeated and used to provide bistabilityresults.

The bistability or image stability results provided in Table 2 wereobtained by driving each sample to either a white, black or red state,disconnecting the sample from the power source, leaving the samples for24 hours, and then measuring the optical state again. The differencebetween in the optical values before and after storage is the deltavalue in Table 2.

As demonstrated by Table 2, image stability of the color electrophoreticmedium may be improved or maintained with increasing concentrations ofpolyisobutylene, and the color performance (Red a) exhibited substantialimprovement despite increased loading of polyisobutylene when thecopolymer was present in the dispersion.

TABLE 2 CS3 Sample 1 Sample 2 Sample 3 Polyisobutylene (wt. %) 0.60%0.20% 0.40% 0.60% PS PE/PP copolymer 0.00% 0.60% 0.40% 0.20% (wt. %)Black and white CR 25 27 26 27 Red L* 31 32 32 32 Red a 43 47 47 49W-BST 24 hrs (ΔL*) 0.5 0.9 0.8 0.5 W-BST 24 hrs (Δa) −0.2 0.0 0.0 0.3K-BST 24 hrs (ΔL*) 0.0 −0.3 −0.4 −0.2 K-BST 24 hrs (Δa) 0.0 0.0 −0.2−0.2 R-BST 24 hrs (ΔL*) 0.7 0.3 0.3 0.5 R-BST 24 hrs (Δa) 0.2 −0.4 −0.10.0

By adding a combination of polyisobutylene and linear diblock copolymerbased on styrene and ethylene/propylene, not only can the imagestability be maintained and/or improved, but also the color state canachieve a better level. Thus, a combination of PIB and a co-polymeraccording to the various embodiments of the present invention providesadditives that can be included in electrophoretic media to improve theperformance of the media.

It will be apparent to those skilled in the art that numerous changesand modifications can be made in the specific embodiments of theinvention described above without departing from the scope of theinvention. Accordingly, the whole of the foregoing description is to beinterpreted in an illustrative and not in a limitative sense.

We claim:
 1. An electrophoretic medium comprises: (a) a non-polar fluid;(b) a plurality of first charged particles; (c) polyisobutylene; and (d)an additive selected from co-polymers of a vinyl aromatic and at leastone substituted or unsubstituted hydrocarbon having 2 to 5 carbons andat least one double bond, wherein the weight percent of styrene in thecopolymer is 20% to 40%.
 2. The electrophoretic medium of claim 1,wherein the vinyl aromatic is selected from the group consisting ofstyrene, alpha-methyl styrene, 2-methyl styrene, 3-methyl styrene,4-methyl styrene, 44-butyl styrene, 4-cyclohexyl styrene, 4-dodecylstyrene, 2,4-diisopropyl styrene, 2,4,6-trimethyl styrene,2-ethyl-4-benzyl styrene, 4-(phenyl butyl)styrene, 1-vinyl naphthalene,2-vinyl naphthalene, vinyl anthracene, 4-methoxy styrene,monochlorostyrene, dichlorostyrene, divinyl benzene, indene, methylindene, and combinations thereof.
 3. The electrophoretic medium of claim1, wherein the vinyl aromatic comprises styrene.
 4. The electrophoreticmedium of claim 1, wherein the at least one substituted or unsubstitutedhydrocarbon is selected from the group consisting of ethylene,propylene, isoprene, butylene, butadiene, and combinations thereof. 5.The electrophoretic medium of claim 1, wherein the additive is aco-polymer of styrene, ethylene, and propylene.
 6. The electrophoreticmedium of claim 1, wherein the weight ratio of polyisobutylene toadditive in the dispersion is 1:5 to 5:1.
 7. The electrophoretic mediumof claim 1, wherein the first charged particles are red, green, blue,cyan, yellow, or magenta.
 8. The electrophoretic medium of claim 7further comprising a plurality of second charged particles dispersed inthe non-polar fluid.
 9. The electrophoretic medium of claim 8, whereinthe second charged particles are white or black.
 10. The electrophoreticmedium of claim 9, wherein the second charged particles are white andthe first and second charged particles have opposite charge polarities.11. The electrophoretic medium of claim 10, wherein the second chargedparticles comprise titania.
 12. The electrophoretic medium of claim 10further comprising a plurality of third charged particles dispersed inthe non-polar fluid.
 13. The electrophoretic medium of claim 12, whereinthe third charged particles are black and the first and third chargedparticles have the same charge polarity.
 14. The electrophoretic mediumof claim 13, wherein the third charged particles comprise, carbon blackor copper chromite.
 15. The electrophoretic medium of claim 1, furthercomprising an ionic surfactant.
 16. The electrophoretic medium of claim1, wherein the combined weight percent of polyisobutylene and additiveis 0.1 to 2.5% based on the total weight of the dispersion.
 17. Theelectrophoretic medium of claim 1, wherein the combined weight percentof polyisobutylene and additive is 0.5 to 1% based on the total weightof the dispersion.
 18. An electrophoretic display comprising anelectrophoretic medium according to claim 1.